The present invention is related to Japanese patent application No. Hei. 11-307491, filed Oct. 28, 1999; the contents of which are incorporated herein by reference.
The present invention relates to a glow plug, and more particularly, to a glow plug having a combustion pressure sensor used as an auxiliary starting device for an internal combustion engine, such as a diesel engine or the like.
A conventional glow plug is disclosed, for example, in Japanese Unexamined Utility model Publication No. 4-57056. The disclosed glow plug includes a cylindrical housing that is mountable in an internal combustion engine. A sheath (pipe member), which receives a heating element, and a rod-like metal central electrode are held in the cylindrical housing. The heating element is heated up upon energization by the central electrode. A piezoelectric element, which outputs an electric signal, in response to a load (pressure) applied to the sheath in an axial direction of the plug, is also received in an interior of the housing.
Furthermore, an O-ring is disposed between the sheath and the housing in the glow plug. When the axial load is applied to the sheath in response to the pressure developed in a combustion chamber, the sheath slides along the housing via the O-ring. As the sheath slides, the corresponding load is applied to the piezoelectric element, and the electric signal corresponding to the load is outputted from the piezoelectric element. An ignition timing at the combustion chamber is determined based on the electric signal.
In the described prior art glow plug, gas tightness of an interior of the housing solely depends on the O-ring, which allows the slide movement of the sheath relative to the housing, so that combustion gas generated in the combustion chamber could penetrate into the interior of the housing. The penetration of the combustion gas into the interior of the housing results in several problems concerning durability of the glow plug. For instance, these problems may include deterioration of the piezoelectric element due to the high temperature of the combustion gas, disconnection of the heating element due to air-oxidation of the heating element, and leakage of output electrical charge from the piezoelectric element, for example, induced by moisture.
Furthermore, since the piezoelectric element, which constitutes the combustion pressure sensor, is arranged within the housing, the housing needs to have an opening, through which a signal output line of the piezoelectric element is extended out from the housing, and a seal for sealing the opening. This results in a relatively complicated wiring structure for extending the output line of the combustion pressure sensor out of the housing.
To overcome the aforementioned drawbacks, the present invention provides a glow plug comprising a cylindrical housing mounted in an internal combustion engine, wherein one end side of the cylindrical housing is positioned at a combustion chamber side of the internal combustion engine. A cylindrical pipe member is held in the housing such that one end side of the pipe member is exposed from the one end of the housing. A heating member is arranged in the pipe member, wherein the heating member is heated up upon energization. A rod-like metal central shaft is received in the housing such that part of the central shaft protrudes from other end of the housing, wherein the central shaft is provided for energizing the heating member. The present invention is characterized in that an internal surface of the housing and an external surface of the pipe member are secured with each other without forming a substantial gap between them at the one end side of the housing. A combustion pressure sensor is arranged around the part of the central shaft, which protrudes from the other end of the housing to measure a combustion pressure in the internal combustion engine based on a force acting on the pipe member upon development of the combustion pressure.
In another aspect of the invention, the combustion pressure sensor is arranged around the part of the central shaft, which protrudes from the other end of the housing. Therefore, the combustion pressure sensor is disposed outside of the housing. As a result, it is not necessary to provide the complicated wiring structure for extending the output line of the combustion pressure sensor out of the housing. As a result, in the glow plug of the present invention, both the gas tightness of the housing interior and the simplification of the wiring structure for the output line of the combustion pressure can be advantageously achieved.
In order to secure the internal surface of the housing and the external surface of the pipe member with each other without forming a substantial gap between them at one end side of the housing, the pipe member can be press fit into the housing, or alternatively, the internal surface of the housing and the external surface of the pipe member can be brazed together.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.